In recent years, an MRAM (Magnetic Random Access Memory) has been developed as a memory using magnetic material. The MRAM uses an MTJ (Magnetic Tunneling Junction) utilizing a tunneling magnetoresistive (TMR) effect as a factor element. The MTJ element has a structure in which a non-magnetic material layer (insulating layer) is sandwiched between two ferromagnetic layers, which allows the magnetization direction of a ferromagnetic layer (recording layer) on one side to be reversed due to an external magnetic field. Thus, in the MTJ element, the magnetization direction of the magnetic material layer is controlled to thereby record information. Because the magnetization direction of the magnetic material does not change even when the power supply is turned off, a non-volatile operation for holding the recorded information can be achieved. To change the magnetization direction of the MTJ element (rewrite information), not only a method for applying a magnetic field from the outside, but also a spin transfer torque magnetization reversal (spin injection magnetization reversal) method for allowing a DC current to flow through an MTJ element to reverse the magnetization have recently been found. For example, Patent Literature 1 discloses an MTJ element which uses an in-plane magnetized material as a recording layer and utilizes the spin injection magnetization reversal, and also discloses a memory having the MTJ element integrated thereon, that is, an SPRAM (SPin-transfer torque Magnetic Random Access Memory).
To improve the integration of the SPRAM, it is necessary to miniaturize the MTJ element. In this case, the thermal stability of magnetic information in the MTJ element is a problem to be solved. When thermal energy generated by an ambient temperature is high with respect to magnetic energy necessary for allowing the recording layer of the MTJ element to be reversed, the magnetization is reversed even when no external magnetic field or current is applied. Because the magnetic energy of the MTJ element decreases along with a reduction in the size, the thermal stability decreases along with the miniaturization of the element. Even in a minute area, it is effective to increase the crystal magnetic anisotropy of the recording layer material of the MTJ element to maintain the thermal stability and achieve highly-reliable operation. Up to now, an MTJ element using a perpendicularly magnetized material having a higher crystal magnetic anisotropy than that of the in-plane magnetized material is disclosed (Patent Literature 2). Further, in the MTJ element using the perpendicularly magnetized material, a demagnetizing field applied in the recording layer affects the direction in which a current density (write current density) necessary for reversing the magnetization is reduced, unlike the in-plane magnetized MTJ element. Thus, there is an advantage of reducing the write current density and suppressing power consumption, compared with the in-plane magnetized MTJ element.
There is disclosed a structure in which magnesium oxide (MgO) is used for an insulating layer (barrier layer) as means for improving a resistance change ratio (TMR ratio) in the perpendicularly magnetized MTJ element, and material (CoFeB or the like) having high electron spin polarizability is disposed on both sides of the insulating layer (Patent Literature 3). Here, the perpendicularly magnetized ferromagnetic layer is disposed in direct contact with the high-polarizability magnetic layer. There is also proposed an element using a structure (synthetic ferri-magnetic structure) in which non-magnetic material layer is sandwiched by two perpendicularly magnetized layers as a perpendicularly magnetized layer (Patent Literature 3). In this case, magnetizations of two perpendicularly magnetized layers are coupled in an antiparallel direction, which provides the effect of suppressing a stray field generated from the perpendicularly magnetized layer.
To produce the perpendicularly magnetized MTJ element as described above and to obtain a higher TMR ratio, the crystalline orientation of each of the barrier layer and the high polarizability magnetic layer formed on both sides of the barrier layer is important. From the past study on the in-plane magnetized TMR element, it is known that a high TMR ratio is obtained when an MgO (001) barrier layer having an NaCl structure is used and a CoFeB layer having a bcc (001) crystal structure is disposed on both sides of the MgO (001) barrier layer. When CoFeB is formed at room temperature, the CoFeB is grown in an amorphous state. When MgO is further formed thereon, MgO (001) crystal is grown. When an anneal treatment is carried out after CoFeB is further formed thereon, the CoFeB layer has a crystalline orientation in bcc (001) with the MgO (001) crystal as a core. In the case of the in-plane magnetized TMR element, the bcc (001) orientation of MgO (001) and CoFeB are realized using such a mechanism.